1. Field of the Invention
The present invention relates to a low-frequency signal optical transmission system and a low-frequency signal optical transmission method, which optically enable a long distance transmission of a low-frequency signal (sine wave) with high stability by compensating a phase delay amount caused by the transmission. In particular, the present invention relates to a low-frequency signal optical transmission system and a low-frequency signal optical transmission method which are suitable for an optical transmission at a low frequency equal to or lower than 20 GHz.
2. Description of the Related Art
FIG. 7 is a diagram illustrating a configuration of a conventional optical transmission system (see, for example, J. Francois and B. Shillue, “Precision timing control for radio astronomy”, IEEE control systems magazine, 19-27, Feb. 2006). Referring to FIG. 7, a laser beam is distributed by an optical coupler 21 on a transmitting side, and two coherent optical signals that are different in wavelength are generated from one of the laser beams thus obtained by a two-light wave generator 22 by using a microwave signal M1. Thus, are two optical signals (wavelengths λ1 and λ2) that are spaced apart by a frequency of the microwave signal M1 are generated. The wavelength λ1 is the same as that of the input laser beam, while the wavelength λ2 is generated by another laser as a signal that is phase-locked to the wavelength λ1. The microwave signal M1 is a highly-stable signal to be transmitted. The two-light wave generator 22 is configured so as to satisfy a condition that the two optical signals be polarized in the same manner.
The two optical signals serving as vertically polarized waves are guided to a polarization beam splitter 23, and pass through a fiber stretcher 24. The two optical signals are further distributed by an optical coupler 7 on a receiving side after passing through an optical fiber 6. One set of the two optical signals are guided to a photodetector 8, and output as a microwave signal M2.
The remaining one set of the two optical signals that have been distributed by the optical coupler 7 are frequency-shifted by a frequency of a microwave signal M3 by an optical modulator 9 as a round-trip signal, and then reflected by a Faraday reflector 10. The Faraday reflector 10 applies 90-degree Faraday rotation to the optical signals, and hence the remaining one set of the two optical signals are reflected as optical signals different in polarization by 90 degrees.
The reflected lights from the Faraday reflector 10 are again frequency-shifted by the frequency of the microwave signal M3 by the optical modulator 9, and then pass through the optical coupler 7, the optical fiber 6, and the fiber stretcher 24 to be returned to the polarization beam splitter 23. In consideration of photoreversibility, the optical signals returned from the receiving side are the optical signals different in polarization by 90 degrees, and hence are horizontally polarized waves. Therefore, the optical signals are guided to an optical coupler 25 by the polarization beam splitter 23.
The remaining one of the optical signals having the wavelength λ1 distributed by the optical coupler 21 and the two optical signals guided by the polarization beam splitter 23 are mixed by the optical coupler 25. The two optical signals output from the polarization beam splitter 23 are different in frequency from the optical signal output from the optical coupler 21 by a frequency twice as high as that of the microwave signal M3. The optical signals mixed by the optical coupler 25 are detected as microwave beat signals by a photodetector 26. A round-trip measurement only for the optical signal having the wavelength λ1 is performed. The beat frequency is the frequency twice as high as that of the microwave signal M3. The beat frequency is multiplied by a mixer 27 by a shift frequency of a microwave signal M4 having a frequency twice as high as that of the microwave signal M3, and an error signal is used for controlling the fiber stretcher 24.
However, the conventional technology raises the following problems. That is, the shift frequency of the microwave signal M3 is provided to distinguish between a transmitted signal and a returned signal, which is a low-frequency signal. A measurement is performed only for a phase of one optical signal, and hence the microwave signal M3 exerts an influence upon a measurement result. Therefore, the microwave signal M3 and the microwave signal M4 having the shift frequency twice as high as that of the microwave signal M3 need to be phase-locked through some method. Further, a disturbance that has occurred during the transmission through the optical fiber 6 exerts an influence upon the measurement result because the measurement is performed only for the phase of one optical signal. In addition, there arises such a problem that an influence of polarization made dispersion (PMD) cannot be removed because the measurement is performed only for the phase of one optical signal.